New York: While male chimpanzees actively challenge their superiors to win higher rank, females accept their position in the social pecking order, waiting until more senior group members die before moving up the ladder, new research has found.
This is despite the fact that high-ranking chimpanzees of both sexes usually have better access to food and mates, boosting chances of survival for themselves and their offspring.
"We found that, after entering the adult hierarchy, there was a complete absence of successful challenges for rank increases among females," said lead author on the study Steffen Foerster, senior research scientist at Duke University in in Durham, US.
"It's like a formal queue," Foerster noted.
To explore how female chimpanzees maneuver up and down the social ladder, the researchers plumbed more than 40 years of daily records documenting the behaviours of 100 or so wild chimpanzees residing in Gombe National Park in Tanzania.
Chimpanzees signal dominance and submission to each other through acts of aggression, such as chases and attacks, and through making a sound called a "pant-grunt," which is a clear sign of subordination to a superior.
The team used a new rating system to document these interactions, allowing them to determine the rank orders of male and female chimpanzees and watch how they shifted over time.
Their results, published in the journal Scientific Reports, showed that, unlike males, whose rank usually peaks when they reach their prime in their early 20s before declining again, female rank gradually increases as they age, and their rank order remains stable throughout their lifetimes.
The tendency of female chimpanzees to "wait their turn" rather than fighting for rank reveals the competing priorities males and females face when ensuring the success of their offspring.
"If a male has a high rank even for a short time but manages to fertilize a lot of females, he achieves high reproductive success," senior author Anne Pusey from Duke University said.
"Whereas a female is only able to raise one offspring at a time, so her reproductive success depends largely on how long she lives," Pusey noted.
Compared to males, female chimpanzees "likely have to consider a long-term strategy," Foerster said.